In the discharge of exhaust steam from an axial flow steam turbine to, for example, an adjacent condenser, the flow of steam therethrough preferably should be relatively smooth. The discharge of the exhaust steam also should minimize energy losses therein from the accumulation of vortices, turbulences, non-uniformities in the flow, and the like.
The exhaust steam from the turbine generally is directed into an exhaust hood. The exhaust steam then passes through an exhaust hood discharge in a direction essentially normal to the axis of the turbine into the condenser or elsewhere. Efficient operation of the steam turbine thus requires a smooth transition from the axial flow through the turbine to the radial flow in the exhaust hood as well as a smooth flow out of the exhaust hood discharge and into the condenser. Achieving a relatively uniform flow distribution at the discharge of the exhaust hood generally provides for an efficient conversion of energy in the turbine and effectively supplies the exhaust steam to the condenser.
Improved efficiency at the later stage buckets of the steam turbine prior to the exhaust generally also requires a relatively uniform circumferential and favorable radial pressure distribution. Diffusers commonly are employed in steam turbines to improve the overall efficiency and output by providing a pressure recovery therein. In conventional exhaust hoods, the maximum pressure recovery generally comes within the diffuser at the end of a steam guide. An amount of the pressure recovery at the end of the steam guide, however, may be lost due to improper area scheduling downstream thereof. Moreover, space limitations may limit the ability of the diffuser to raise the static pressure as the steam velocity is reduced by increasing the flow area. Specifically, attempts have been made to accomplish these efficiency goals while employing an exhaust hood having as short an axial length as possible so as to limit the length of the rotor. Although a reduced rotor shaft length may reduce production costs such also may result in a reduced available area ratio at the end of a steam guide. The loss of an amount of the pressure recovery thus means a loss of low pressure section performance and hence overall efficiency.
There is thus a desire for an improved diffuser for an exhaust hood of a steam turbine. The diffuser preferably provides increased performance and efficiency via an increased available area ratio while also reducing production costs with respect to the length of the rotor and otherwise.